1. Field of the Invention
The present invention relates to a drive shaft assembly for use in a machine which drives a rotating work piece at an operating speed above the critical speed with rotating parts. It particularly relates to a drive shaft assembly having flexability for permitting the work piece to rotate about its center of mass under conditions of imbalance.
It has been recognized for some time that rotating machinery should be well balanced since any unbalance would result in undue vibration which in turn has a deleterious effect on the bearings and journals. For example, in U.S. Pat. No. 2,344,753 there is disclosed a balancing machine for propeller shafts. The machine utilizes a cantilever bearing for supporting the part to be balanced. The bearing is mounted in an inertia member damped by suitable springs such that the cantilever bearing can vibrate independently of the inertia member and such independent movement is transmitted to an indicating device. It is taught that such an arrangement provides a natural frequency for the bearing and inertia member which is out of the range of the frequency of the vibration produced by any unbalance in the propeller shaft.
It also has been proposed to accommodate radial deflections which occur from imbalance through the use of a flexible bearing support. U.S. Pat. No. 3,388,880 describes a flexible bearing support designed such that the deflections of the supporting means cannot be transferred to the bearing itself. Rather, the bearing support has a flexible web portion which is adapted to be deformed to compensate for such deflection.
U.S. Pat. No. 4,129,343 describes a unitary self-aligning bearing assembly of a ball and socket type in which the outer radial bearing ring has at least one radial end face carrying a bearing race with an axial thrust bearing mounted thereon.
As higher rotational speeds are utilized, for example in turbines and compressors, the operational speeds frequently are above the critical speed of the rotating parts. Thus in U.S. Pat. No. 4,324,440 there is described a containing bearing for use with rotors operating above their critical speed. There is provided a gap between the containing bearing and the rotor. The gap is equal to or larger than the maximum amplitude of vibration of the rotor in normal operation. The containing bearing is supported by a resilient sleeve which is fixed to prevent its rotation.
The problem of imbalance is particularly acute in spray dryers which utilize an atomizer disc. The performance of a spray dryer used in a flue gas cleaning system can be optimized by use of an atomizer disc to produce droplets of a scrubbing solution or slurry which are correctly sized for both ballistic and chemical reaction requirements. This is achieved by rotating a disc or wheel, similar to a centrifugal pump, at high speeds so the liquid is shattered when injected into the relatively stationary gas.
One of the major problems occurring during operation of such a device is the onset of mechanical vibration caused by unbalance of the atomizer disc. This unbalance is usually the result of uneven buildup (or removal) of scrubbing solution products in or on the surfaces of the disc. Buildup on the disc is normally a gradual process and does not necessarily result in unbalance, while the loss of such buildup from centrifugal forces is normally a step function. Erosion of the wheel, if such occurs, is very slow and can normally be detected. Loss of wear-resistant material (normally ceramics) by fracture and expulsion will also produce a step function unbalanced condition. Any unbalance will produce reactive loads on the atomizer drive shaft and support bearings which will lead to either a very rapid failure or drastic reduction in the life of these components. Clearly, there is a need for a drive shaft assembly which could operate continuously and accommodate a reasonable amount of unbalance at high rotational speeds of, for example, 10,000-30,000 rpm.